The present invention relates to a catalyst for steam cracking reactions and the related preparation process.
The most widely-used method for the production of light olefins, in particular ethylene and propylene, is the steam cracking process, in which a hydrocarbon charge is heated, in the presence of water vapor, in specific ovens to produce a gaseous stream rich in olefins. Steam cracking is a thermal process which is carried out on an industrial scale without catalysts. The setting up of a catalytic system which allows an increase in the yields to the desired products would provide important advantages; owing to the large volumes of products in question (for example the world-wide production of ethylene is over 70 Mton/year), even small percentage increases in the yield would have a great impact on process economy.
The use of catalysts for steam cracking reactions has not been widely studied, even if various companies and research groups have occasionally worked in this area since the 70s"". In some cases a process has been defined but industrial applications are not known at the moment.
Among the most significant references are the following, which identify calcium-aluminate compounds in which the 12CaO.7Al2O3 (mayenite) phase prevails, as the most active materials for the catalysis of naphtha cracking:
A. A. Lemonidou, I. A. Vasalos, Applied Catalysis, 54 (1989), 119-138;
A. A. Lemonidou, I. A. Vasalos, Proc. 1987 AIChE Spring National Meeting, Houston, Mar. 29-Apr. 2, 1987;
K. Kikuchi, T. Tomita, T. Sakamoto, T. Ishida, Chemical and Engineering Progress, 81 (1985) 6, 54.
B. Basu, D. Kunzru, Industrial and Engineering Chemistry Res., 1992, 31, 146-155.
Another reference has also demonstrated the good performance of materials consisting of Ca-aluminate mixtures:
S. Nowak, G. Zimmermann, H. Gushel, K. Anders, in xe2x80x9cCatalysis in Petroleum Refining 1989xe2x80x9d (D. L. Trimm et al. Eds.), Elsevier Science Publishers B.V., 1990.
As far as studies relating to industrial development are concerned, mention can be made of Asahi Chemical which claims a process, almost ready for commercialization, for steam cracking in a circulating bed, using a catalyst based on ZSM-5 and ZSM-11 zeolites, charged with metals such as Fe, Mg and/or Ib metals. This process partially increases the yield to ethylene, but the reaction is mainly directed towards the production of propylene and aromatics. Recent information (PERP Report 96/97S12xe2x80x94Chem Systems, September 1997) reveals that the process still has several problems of a technological nature to be solved, among which many aspects relating to the catalyst (activity, regeneration, duration), before it can be actually commercialized. More or less the same situation also applies to the Russian process of Vniios (Research Institute for organic syntheses), which uses potassium vanadate supported on corindone/mullite as catalyst, with the addition of promoters. Exxon has patented a process using an inert solid as heat transporter or catalysts based on mixed oxides of Mg, Ca, Mn, Be, Sr, Ce, V, Cs (W. Serrand et al., WO 97/31083). This process however is preferably designed for heavy charges (e.g.  greater than 500xc2x0 C.) and comprises, in fact, a particular type of horizontal moving bed reactor with two rotating screws which help the movement of the charge.
A technology which seems closer to a possible industrial application is the Pyrocat process, set up by Veba Oel and Linde (M. Wyrosteck, M. Rupp, D. Kaufmann, H. Zimmermann, Proc. 15th World Petroleum Congress, Beijing, Oct. 12-16, 1997). This technology comprises implementation of steam cracking plants without modifying the design of the ovens. The idea is based on coating the inside of the cracking tubes with a solid layer having a catalytic effect and which inhibits the formation of coke, thus prolonging the times between subsequent stoppages for decoking operations. The catalyst is based on Al2O3/CaO and contains, as gasification promoter, compounds of alkaline metals. The technology however can only be applied to conventional cracking plants, operating with conventional charges.
It can therefore be seen from literature that catalysts based on calcium aluminates can be used in steam cracking reactions for the production of ethylene and propylene. The calcium aluminates which can be formed are the following, in increasing order of calcium content: CaO.6Al2O3, CaO.2Al2O3, 3CaO.5Al2O3, CaO.Al2O3, 5CaO.3Al2O3, 12CaO.7Al2O3, 2CaO.Al2O3 and 3CaO.Al2O3 but it is not disclosed in literature which is the preferred crystalline phase for steam cracking reactions. In fact, according to Lemonidou (A. A. Lemonidou, I. A. Vasalos, Applied Catalysis, 54 (1989), 119-138) the most effective catalyst is a mixture of calcium-aluminates in which the prevalent compound is mayenite (12CaO.7Al2O3); S. Nowak, on the other hand, has patented a catalyst (DD-243 647 of 1987) in which the preferred phases have a lower content of calcium oxide: CaO.Al2O3 and CaO.2Al2O3.
The preparation of these catalysts is generally effected by the mechanical mixing of the oxides or their aluminum and calcium precursors and subsequent calcination at a high temperature. This process generally leads to the formation of materials in which there are several phases, even if in some cases one phase may be distinctly prevalent with respect to the others. No information is provided however in scientific literature with respect to this type of catalyst, on the production of pure calcium-aluminate materials by means of the syntheses described.
We have now found a process for obtaining pure mayenite (12CaO.7Al2O3), which surprisingly allows better results to be obtained in terms of yield to light olefins in the field of naphtha steam cracking reactions with respect to mixtures containing mayenite and other calcium-aluminates either pure or mixed with each other.